The present invention relates to improvements within a hydraulic pressure modulating control circuit and more particularly to such a control circuit wherein a modulating valve is manually operable for precisely regulating fluid pressure communicated to a master clutch.
The invention is described herein with particular reference to a transmission of the type including a number of range clutches, speed ratio clutches, and directional clutches which may be selectively engaged to transmit driving force through the transmission. Although the following description refers to friction clutches for establishing a gear train within the transmission, other transmissions of a type contemplated by the present invention may include brakes for establishing a selected gear train. Accordingly, the following description and claims refer to the use of friction clutches and that term is intended to refer to either friction clutches or friction brakes employed to establish a selected gear train within a transmission.
The present invention also particularly contemplates such a transmission having one set of master clutches which are of relatively and durable construction for establishing a selected drive train in the transmission. The master clutches may be engaged sequentially after other clutches within the transmission in order to absorb the shock of transmitting power through the drive train of a vehicle. Preferably, a speed ratio clutch and a range clutch are engaged prior to engagement of a directional clutch within the transmission described below, the directional clutch thus serving as a master clutch.
The present invention contemplates a transmission control circuit employing a differential valve and an associated pressure modulating valve similar to those described in U.S. Pat. No. 3,468,194, which issued on Sept. 23, 1969 and was assigned to the Assignee of the present invention.
However, the differential valve and associated pressure modulating valve, as described below, preferably function in combination with a manual modulating valve, that combination also being set forth within a co-pending application entitled: TRANSMISSION CONTROL SYSTEM WITH DUAL PURPOSE MODULATING VALVE, filed Apr. 27, 1973 as application Ser. No. 355,191, now U.S. Pat. No. 3,882,980, issued May 13, 1975, and assigned to the Assignee of the present invention.